1. Field of the Invention
The present invention relates to a transformer, which is formed by laminating sheets with spiral conduction patterns and sintering it.
2. Description of the Prior Art
FIG. 8 is an exploded perspective view showing a usual flat type transformer which has a multi-layered coil block.
A transformer 21 contains an insulating sheet 22 made from ceramic material. At the both sides of the sheet 22, spiral conduction patterns 22a and 22b are formed, and spiral conduction patterns 22a and 22b form the primary coil. Similarly, at the both sides of the sheet 23, spiral conduction patterns 23a and 23b are formed, and spiral conduction patterns 23a and 23b form the secondary coil. In the center of these sheets 22 and 23, a hole is made to insert a core. And into the holes of sheets 22 and 23, legs 24a, 24a of the cores 24, 24 are inserted. Lead terminals are connected to the primary coil and the secondary coil, and these lead terminals are used in connecting the coils to the external circuit.
In the transformer 21, a through hole 22c is made in the sheet 22, and the conduction pattern 22a and the conduction pattern 22b are connected through the through hole 22c. Thus, the primary coil with necessary windings is formed. The primary coil is connected by soldering etc. to the external terminals (not shown in the drawing). At this time it is easier to connect the primary coil if both ends of the primary coil are on the same surface. In order to accomplish this, the end of the conduction pattern 22b is led through a through hole 22e to the other side of the sheet 22. Also, the ends of the conduction patterns 22a and 22b are placed on the same surface of the sheet 22, and are led to the edge of the sheet.
Similarly, conduction patterns 23a and 23b are connected through a through hole 23c. Also, the conduction pattern 23b is led through a through hole 23e to the other side of the sheet 23. The the ends of the conduction patterns 23a and 23b are placed on the same surface of the sheet 23 and are led to the edge of the sheet.
Between the sheet 22 and the sheet 23 and on both sides of the sheets 22 and 23, sheets 25 are placed for insulation purposes. By laminating, pressing and sintering the necessary number of sheets, a multi-layered coil block is produced. On the edges of the multi-layered coil block, the ends of the conduction patterns 22a, 22b, 23a and 23b are exposed. Lead terminals are connected to these exposed ends of the conduction patterns.
As for the sheet material used in the transformer, for example, ceramic material such as the alumina ceramic material or dielectric ceramic material is used. Also, as the conduction pattern material, for example, conducting paste made from good conductive metal such as Ag or Cu is used.
However, since this type of transformer uses the sheet material made from ceramic material, the high temperature sintering is required in producing the multi-layered coil block. As the result, there are problems with oxidation and evaporation of Ag or Cu in the conduction pattern material. In such circumstances, the direct current resistance increases in the coil, resulting in lowering of the transformer efficiency. Also, since the connection between a transformer and the external circuits is accomplished by lead terminals, the assembly process of attaching terminals is complex. Thus, productivity is low, and the cost of production is high. Furthermore, it limits the miniaturization of a transformer.